diff --git a/src/workerd/api/global-scope.c++ b/src/workerd/api/global-scope.c++
index 2a98eb1f8a9..898ffd8761f 100644
--- a/src/workerd/api/global-scope.c++
+++ b/src/workerd/api/global-scope.c++
@@ -248,6 +248,23 @@ kj::Promise<DeferredProxy<void>> ServiceWorkerGlobalScope::request(
     body = Body::ExtractedBody(jsStream.addRef());
   }
 
+  // If the request doesn't specify "Content-Length" or "Transfer-Encoding", set "Content-Length"
+  // to the body length if it's known. This ensures handlers for worker-to-worker requests can
+  // access known body lengths if they're set, without buffering bodies.
+  if (body != nullptr &&
+      headers.get(kj::HttpHeaderId::CONTENT_LENGTH) == nullptr &&
+      headers.get(kj::HttpHeaderId::TRANSFER_ENCODING) == nullptr) {
+    // We can't use headers.set() here as headers is marked const. Instead, we call set() on the
+    // JavaScript headers object, ignoring the REQUEST guard that usually makes them immutable.
+    KJ_IF_MAYBE(l, requestBody.tryGetLength()) {
+      jsHeaders->setUnguarded(jsg::ByteString(kj::str("Content-Length")),
+                               jsg::ByteString(kj::str(*l)));
+    } else {
+      jsHeaders->setUnguarded(jsg::ByteString(kj::str("Transfer-Encoding")),
+                               jsg::ByteString(kj::str("chunked")));
+    }
+  }
+
   auto jsRequest = jsg::alloc<Request>(
       method, url, Request::Redirect::MANUAL, kj::mv(jsHeaders),
       jsg::alloc<Fetcher>(IoContext::NEXT_CLIENT_CHANNEL,
diff --git a/src/workerd/api/http-test.js b/src/workerd/api/http-test.js
index 0a34a47cc01..31466a6a7b9 100644
--- a/src/workerd/api/http-test.js
+++ b/src/workerd/api/http-test.js
@@ -7,12 +7,50 @@ import assert from "node:assert";
 let scheduledLastCtrl;
 
 export default {
+  async fetch(request, env, ctx) {
+    const { pathname } = new URL(request.url);
+    if (pathname === "/body-length") {
+      return Response.json(Object.fromEntries(request.headers));
+    }
+    return new Response(null, { status: 404 });
+  },
+
   async scheduled(ctrl, env, ctx) {
     scheduledLastCtrl = ctrl;
     if (ctrl.cron === "* * * * 30") ctrl.noRetry();
   },
 
   async test(ctrl, env, ctx) {
+    // Call `fetch()` with known body length
+    {
+      const body = new FixedLengthStream(3);
+      const writer = body.writable.getWriter();
+      void writer.write(new Uint8Array([1, 2, 3]));
+      void writer.close();
+      const response = await env.SERVICE.fetch("http://placeholder/body-length", {
+        method: "POST",
+        body: body.readable,
+      });
+      const headers = new Headers(await response.json());
+      assert.strictEqual(headers.get("Content-Length"), "3");
+      assert.strictEqual(headers.get("Transfer-Encoding"), null);
+    }
+
+    // Check `fetch()` with unknown body length
+    {
+      const body = new IdentityTransformStream();
+      const writer = body.writable.getWriter();
+      void writer.write(new Uint8Array([1, 2, 3]));
+      void writer.close();
+      const response = await env.SERVICE.fetch("http://placeholder/body-length", {
+        method: "POST",
+        body: body.readable,
+      });
+      const headers = new Headers(await response.json());
+      assert.strictEqual(headers.get("Content-Length"), null);
+      assert.strictEqual(headers.get("Transfer-Encoding"), "chunked");
+    }
+
     // Call `scheduled()` with no options
     {
       const result = await env.SERVICE.scheduled();
diff --git a/src/workerd/api/http.c++ b/src/workerd/api/http.c++
index 4e1055979b2..caac5a6c266 100644
--- a/src/workerd/api/http.c++
+++ b/src/workerd/api/http.c++
@@ -284,6 +284,10 @@ bool Headers::has(jsg::ByteString name) {
 
 void Headers::set(jsg::ByteString name, jsg::ByteString value) {
   checkGuard();
+  setUnguarded(kj::mv(name), kj::mv(value));
+}
+
+void Headers::setUnguarded(jsg::ByteString name, jsg::ByteString value) {
   requireValidHeaderName(name);
   auto key = toLower(name);
   value = normalizeHeaderValue(kj::mv(value));
diff --git a/src/workerd/api/http.h b/src/workerd/api/http.h
index b5c9f9a52b4..0d9573b2e92 100644
--- a/src/workerd/api/http.h
+++ b/src/workerd/api/http.h
@@ -95,6 +95,9 @@ class Headers: public jsg::Object {
   // is not permitted to be combined into a single instance.
   bool has(jsg::ByteString name);
   void set(jsg::ByteString name, jsg::ByteString value);
+  void setUnguarded(jsg::ByteString name, jsg::ByteString value);
+  // Like set(), but ignores the header guard if set. This can only be called from C++, and may be
+  // used to mutate headers before dispatching a request.
   void append(jsg::ByteString name, jsg::ByteString value);
   void delete_(jsg::ByteString name);
   void forEach(jsg::Lock& js,